Cracking the secret of holes in Antarctic ice

16 June 2019

MELTING-ICE

Melting Ice

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Holes, thousands of square kilometres in size, were found in the Antarctic ice in 2016 and 2017. The formation of these holes affects how heat is distributed in local ecosystems, carbon release from the region, and ultimately global climate. But why did they form in the first place?

By harnessing data from the Southern Ocean Carbon and Climate Observations and Modeling project (SOCCOM), weather stations, decades of satellite images, and even elephant seals, PhD student Ethan Campbell and his colleagues at the University of Washington took advantage of the events of 2016 and 2017 to assemble a model of how these holes, known as polynyas, appear.

“We realised that there are a combination of factors that have to occur for one of these polynyas to open up,” explains Campbell.

The factors turn out to be a trifecta consisting of ocean salinity, hurricane-force winds, and differences in ocean water temperature. Salinity, or salt concentration, affects the density gradient of water beneath the ice. Fresh water is less dense than salty water, so it tend to float on top. But unlike a mixture of oil and water, which behaves the same way, the density difference between saltier and fresher water tends to be quite weak, so it still allows water to mix, releasing heat trapped in the deep ocean. This warms the surrounding surface water just enough to melt the ice barrier, forming a hole.

Once the hole has formed, there is one last crucial piece to the puzzle as described by Campbell. “Storms cause a lot of mixing as well. They mix up saltier, warmer water from below and they kickstart that larger, deeper vertical circulation.” When added together, the trifecta creates a closed loop of continuous water circulation, or vertical circulation. The constant movement of the water prevents the area from freezing shut in a vicious cycle that can last up to 3 years.

How these large expanses of open water form has been a mystery to science for decades. Not only how they form, but the effects the polynyas have on the surrounding environment. And there may be another effect with global consequences. A phenomenon known as Antarctic Bottom Water, traps super-cold, dense water on the seafloor where it acts as a long-term organic sponge accumulated over centuries from the carbon sequested by deceased sea life.

“There’s a lot of carbon locked up in the deep ocean, stored away for centuries normally, and it’s stuck there,” says Campbell.

But the strong currents created by the water circulation in polynyas can pull this deep water to the surface. In the process, the centuries-old buildup of carbon becomes “unstuck” and is released into the atmosphere, potentially accelerating climate change.

There are other consequences of an ice-free patch of ocean surface too. According to Earle Wilson, also from the University of Washington, “the main repercussion of having a large polynya is the effect it has on sea ice itself. By definition, a polynya is just the lack of ice and once you remove the ice, it changes how reflective the surface is. It changes the physical characteristics of the region, so many lifeforms and different ecosystems depend on the sea ice.”

In the years to come, the team will continue their work with SOCCOM to realise their goal of maintaining quality data acquisition of the Antarctic region, which has only just recently become heavily monitored. They'll also be looking in more detail at the data collected in 2016 and 2017 to further refine their theories and models to better predict overall global impact.

“The state of the southern ocean, in particular around Antarctica, that has a huge impact on the state of the global climate and global climate predictions,” concludes Wilson. “So understanding polynyas is a really critical part of making accurate projections of climate.”

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